Due to an explosion in both civilian and military wireless communication, there is a growing need for high speed, reliable, secure, wireless communication of large amounts of data between communicating nodes. It should be noted that the term “wireless” is used throughout this disclosure to refer to any communication that does not depend on a physical link between sender and receiver. Hence, the term “wireless” as used herein excludes fiber optic communication as well as communication over copper wires.
Traditional communication by wireless radio frequencies suffers from several shortcomings, many of which arise from the wide geographic dispersion of typical radio emissions. Even when directional antennae and antenna arrays are used, radio signals are generally disbursed over large geographic areas, causing rapid attenuation of the signal strengths with distance, and also causing the signals to be relatively easy to intercept by unintended receivers. Due to the geographic overlap of radio communication signals, it is typically necessary to assign radio transmissions to specific frequency bands, which are often in limited supply. Furthermore, it is relatively easy for hostile antagonists to attempt to jam radio communications by transmitting radio signals at high energies that blanket a region of interest.
There are several approaches that attempt to address these problems of wireless radio communications. For example, bandwidth restrictions can be mitigated by opportunistically seeking and using bands that are nominally assigned to other uses, but are not currently in use. Various time and coding schemes can be employed to allow more than one communication link to share the same frequency band. And so-called “multi-user” detection can be employed to further distinguish signals transmitted on overlapping frequencies.
The geographic range of wireless signals can be extended by implementing signal relay nodes within a region of interest.
Security of wireless radio communications can be improved, for example, by employing secure transmission methods such as frequency “hopping,” by adding pseudo-noise to communications, and by encoding communications with sophisticated, virtually impregnable cyphers. The Link 16 protocol is an example of this approach.
Nevertheless, all of these approaches to radio communication include significant disadvantages, such as increased cost and complexity, and message processing overhead that can slow communication and limit data transfer speeds.
Laser communication offers an attractive wireless alternative to radio communication, especially when point-to-point communication is required, because the non-dispersed, focused character of laser communication intrinsically avoids most of the problems that are associated with radio communication. In particular, there is no need to assign frequency bands to laser communication users, because interference between laser signal beams is avoided so long as two beams are not directed to the same recipient. Laser signals experience very little attenuation as a function of distance, because the signal energy remains tightly focused in a beam. And communication security is intrinsically high, because interception of and interference with laser communications requires direct interception of a laser communication beam, and/or focusing jamming beams directly at an intended signal receiver.
One important application that can benefit significantly from laser communication is satellite communications, where line-of-sight access is generally available, and where the communication distances are very great. Laser communication can provide communication data rates for satellites that are much higher than radio data rates, with unmatched anti-jam characteristics and an inherently low risk of communications intercept. Laser communication also eliminates the need for frequency planning and authorization, and circumvents the highly congested RF spectrum bandwidth constraints that limit the practical data rates available to users of RF links.
With reference to FIG. 1, laser communications holds great promise for multi-Gbps (Giga-bits per second) connections between space platforms 100, as well as between ground-based nodes 102 and space platforms 100, owing to the availability of efficient, multi-watt laser sources and exceedingly high antenna gain, having beam widths of only 10-20 micro-radians, and telescope apertures that are only four to eight inches in diameter. And even when much lower data rates of tens to hundreds of mega-bits per second (Mbps) are of interest, laser communication may be desirable due to its Low Probability of Intercept (LPI), Low Probability of Detection (LPD) and anti-jam communications link security.
It should be noted that the disclosure herein is mainly presented with reference to satellite communication. However, it will be understood by those of skill in the art that the present disclosure is not limited to satellite communication, but also applies to other implementations of laser communication.
Of course, there are certain problems associated with laser communication that arise specifically from the focused nature of laser beams. In particular, it is necessary for communicating nodes to identify each other and align their lasers so as to effectively communicate. In the case of satellite laser communication, these identification and alignment problems are especially acute, because laser sources that are well separated by terrestrial standards, for example several miles apart from each other, may nevertheless appear to be almost geographically overlapping from the viewpoint of a satellite. Furthermore, thermal and other effects of the atmosphere can lead to angular (apparent location) shifting of an incident laser communication beam, even after it is identified and aligned. These angular vibrational effects, together with short-term mechanical instabilities of the satellite or other receiving node, are referred to herein collectively as “jitter.”
With reference to FIG. 2, there are at least four steps that must be accomplished so as to establish and maintain laser communications. First, a candidate light source, referred to herein as a “hotspot,” must be identified from within a scene of interest 200. Second, the hotspot must be verified as being a laser communication signal, and its transmission source must be identified so as to determine if it is a signal of interest 202. Third, the optics of the laser communication receiving system must be aligned with the incoming beam 204, and finally, once communication has been established, the beam must be tracked during communication, so that the alignment of the “centroid” of the beam with the signal receiving sensor is maintained and the communication is not interrupted 206.
In addition to jitter, tracking of a beam that is a signal of interest can be made even more difficult if there are other hotspots located within the scene of interest, especially if some of these other hotspots are stronger than the signal of interest, and/or located geographically close to the transmitting node of the signal of interest. The problem is made even worse if these other, competing hotspots vary in amplitude over time, possibly even appearing and disappearing.
Accordingly, there is a need for a more reliable and accurate apparatus and method for identifying, aligning, and tracking a beam that is carrying a laser communication signal of interest, including under conditions where other, possibly stronger hotspots are present within the scene of interest that may vary in amplitude, possibly even appearing and disappearing as a function of time.